In patients with idiopathic alveolar proteinosis, the alveoli are filled with materials rich in surfactant components, especially surfactant protein A (SP-A). The anomaly could be caused by either increased secretion, decreased clearance, or both. To clarify this point, we studied five patients who underwent therapeutic lavage and then were ventilated mechanically for 24 h. During the first 8 h of mechanical ventilation, a surfactant-depleted lung was lavaged at selected intervals, and the bronchoalveolar lavage fluid was analyzed. We observed that, after lavage, various surfactant components accumulated in the airways with different time courses. We also observed that SP-A increased until the second hour and then dropped rapidly, suggesting the existence of an efficient mechanism of removal. These findings suggest that idiopathic alveolar proteinosis might be caused by a primary defect in a slow mechanism of removal or by the presence of factor(s) that interfere with the clearance of surfactant and that can be removed by lavage. It seems clear, however, that an increased secretion rate is unlikely to be the major cause of idiopathic alveolar proteinosis.